By Miriam Felton-Dansky
By Lilly Lampe
By R. C. Baker
By Tom Sellar
By Alexis Soloski
By Molly Grogan
By R. C. Baker
Mallett's theory, which steers clear of stargazing, may seem the most down-to-earth option, even though its practicality is as yet inconclusive. The UConn prof proposes that a time warp could be created with a circulating laser beam: "It's not just mass that can cause a bending of space-time. Anything that has energy can do that. So rather than deal with massive objects, where you have to worry about inertial forces, I thought, Why not look at circulating light?" Using a ring laser to produce a continuously rotating beam, Mallett calculated that at high enough intensities, space and time could be twisted in the circle within. "Of course the ideal result for an experiment would be if you turned on the circulating light and saw a little particle coming out that you hadn't put in yet," he adds with a laugh. "At the moment we're just hoping to prove the twisting of space." The Mallett machine might resemble "a long light cylinder"a feature that the lifelong Trekkie says will no doubt please sci-fi aficionados. Davies is skeptical but intrigued: "There are gravitational effects associated with the setup he describes, but whether they achieve a twisting of time, I wouldn't like to say. Stranger things have happened."
The science of time travel has shed its air of outer-limits disrepute for the most part (an institution called the Time Travel Research Center, which claims to have a lab in Long Island, did not respond to requests for an interview), but there remains a hint of sheepishness within academia. Some physicists don't talk of time machines, preferring the technical term "closed time-like curves." Science fiction may have popularized time travel, but in providing abundant logic-shredding inconsistencies, it also bred deep skepticism. Wells himself was certainly mindful of the concept's fundamental counter-intuitiveness. When his protagonist returns from A.D. 802,701, grimy and tousled, a stunned dinner guest exclaims, "A man couldn't cover himself with dust by rolling in a paradox, could he?" The act of historical interference that proves potentially self-nullifying has become a pop-culture commonplacefrom Back to the Future, where Marty McFly finds himself slowly disappearing after his teenage mother takes a sexual interest in him, to the Halloween episode of The Simpsonswhere Homer is transported to the prehistoric era via defective toaster and screws up evolution by sitting on a fish.
This oxymoronic vicious cycle is often dubbed the grandfather paradox. As in, if you go back in time to kill your grandfather, you wouldn't have been born, and wouldn't have been able to go back in time, and your grandfather would still be alive . . . Two diametrically opposed theories are commonly invoked as solutions. The first, the principle of self-consistency, argues that the time traveler was always part of the past. He fails to change it because he did fail; any attempt to do so is futile. The many-worlds theory permits free will, but its implications are no less disconcerting. Some scientists (notably Oxford physicist David Deutsch) propose a model in which a paradox bifurcates reality into the relevant alternate scenarios. Outlandish as it sounds, this hypothesis stems from the microworld of quantum mechanics (where, per Heisenberg, the behavior of subatomic particles is known to be undeterministic). All possible permutations of world history exist, alongside each other. Kaku says the latter conjecture is the dominant one among cosmologists: "When the universe was born, it was smaller than an electron, which is a quantum object that can exist simultaneously in many states. So the universe must also be a quantum object and exist in many states. Big bangs are happening all the time, even as we speak."
Time travel is so rife with paradox (Gott's book even uses Escher drawings to illustrate several concepts) that some physicists and philosophers argue these self-contradictions automatically rule it out. Nahin's Time Machines offers several examples of apocalyptic sci-fi where a paradox represents the ultimate violation of nature and results in the universe being snuffed out. (He cites a "cosmic disgust theory" from John Varley's novel Millennium, in which an attempted intervention prompts the wrathful creator to pen a disapproving note: "If you're going to play games like that, I'll take my marbles and go home. Signed, God.") Instead of inspiring reverent fear before an almighty force, though, time-travel narratives increasingly bespeak a laissez-faire wish-fulfillmentwitness their Reagan-era popularity. Hollywood chrono-trip arrogance reached its apex of entitlement in 2000's Frequency: The hero, sending messages into the past by radio waves, doesn't stop tampering until everything is exactly as he pleases.
Beyond the obvious logical dilemmas, the possibility of time travel uncorks distressing quandaries that range from the ethical to the theological. "Even scientists who acknowledge that you can't rule out travel to the past would nevertheless like there to be a sort of meta-rule that would prevent it," says Davies. "But we know of no such principle." Stephen Hawking has proposed a "chronology protection conjecture"an as-yet-unknown law of physics that would preserve causality and safeguard history from meddlers. (Jean-Claude Van Damme had another name for it: Timecop.)
Dissenters often point to the absence of time travelers in our midst. Why don't we see tourists from the future descending on historic events? A sci-fi buff might point out that any self-respecting temponaut would (duh) show up in disguise. An astrophysicist would simply respond that no time machines have been built yet, and most of the blueprints we have for them stipulate that you can't travel back to an era before the device was first switched oni.e., before the "closed time-like curve" was formed. If a time machine were built tomorrow, we might suddenly find ourselves surrounded by visitors from the future. But any moment in the past, up until today, would be forever off-limits.